Laurent Billon

Hierarchical structures based on self-assembled diblock copolymers within honeycomb micro-structured porous films

This article details the preparation of hierarchically ordered microporous films using the so-called breath figure approach combined with the self-assembly of well-defined poly(n-butyl acrylate)-block-polystyrene or poly(tert-butyl acrylate)-block-polystyrene copolymers synthesized by nitroxide-mediated polymerization. The first level of organization was the hexagonal pattern of pores at the micrometre length scale leading to iridescence properties and surface hydrophobicity of the honeycomb structured films. Optical microscopy with the corresponding 2-Dimensional Fast Fourier Transform highlighted the ordering of the pores over a large scale (∼1 cm2). The second level of structuring was provided by the diblock copolymers chosen for their ability to self-assemble into ordered nanophases. The nanoscale morphology of both the honeycomb films and the corresponding thermally annealed continuous films was systematically investigated by atomic force microscopy (AFM) and small angle neutron scattering (SANS). The film characterization revealed a nanostructuration of the acrylate-based coil–coil diblock copolymer within the walls of the highly ordered microporous films obtained via a simple solvent evaporation method under humid atmosphere. The four synthesized diblock copolymers exhibited different macromolecular features with respect to the Flory–Huggins interaction parameter, the glass transition temperature of each block and the weight fraction of each monomer that influenced the quality of either the micropores structuration or the nanophase segregation.

A versatile route to functional biomimetic coatings: ionomers for honeycomb-like structures

Highly ordered films were prepared by spreading out carbon disulfide (CS) solutions of ionomers over inorganic surfaces and for the first time over an organic surface. These ionomers, hydrophobic homopolymers or copolymers end-capped by a cationic group, were easily synthesized in a one-step reaction by a control radical process called nitroxide-mediated polymerization. Optical and electronic microscopy observations of the films lead to the definition of new criteria for the formation of such structures. Firstly, the effect of a solid support on the honeycomb structure regularity was studied. By comparing two polar substrates, it was found that a more regular organization is obtained over mica than over glass. This phenomenon may be due to the electrostatic interactions between cationic ionomer ends and oxanions of the mica surface. Moreover, for the first time, highly ordered hexagonal patterns were also created on a flexible and soft polymeric surface, a poly(vinyl chloride) sheet. Secondly, it was found that the characteristics of the polymeric chain of the ionomer and mainly its glass transition temperature play an important role in the process of formation of ordered structures. The iridescent aspect of the films, due to light diffraction and optical interferences, was studied and quantified by spectrogoniometry. Finally, results of measurements of superficial tension are discussed; the presence of a honeycomb film at the surface of a material very significantly enhances its hydrophobicity. All these criteria of highly ordered structures correlated to a water repellency and iridescent behaviours lead to a functional biomimetic material.

Effect of the synthetic methodology on molecular architecture: from statistical to gradient copolymers

Styrene-butyl acrylate (S-BuA) copolymers were synthesized by nitroxide-mediated controlled radical polymerization using an alkoxyamine as an initiator. Using different synthetic methodologies, statistical copolymers were be obtained by batch nitroxide mediated polymerization while the gradient composition was a forced gradient by continuous addition of S during BuA polymerization (semi-batch process). These gradient copolymers have been studied by H NMR and size exclusion chromatography to characterize the gradient composition molecular structure. The evolution of the composition was correlated with the glass transition temperature () of the copolymers. The gradient copolymers exhibit one with a value in between the of polystyrene and poly(butyl acrylate), indicating that the materials did not present well defined microphase separation. Specific organization at the air-polymer interface of such copolymers has also been demonstrated by comparison between classical and attenuated total reflection (ATR) Fourier transform infra-red spectra. This bulk soft matter assembly was confirmed by AFM analysis, which showed a different morphology at the surface and in the bulk following removal of the top layer. Moreover, for the most well defined gradient composition, a specific nano-structuring was demonstrated by small angle neutron scattering. The preliminary rheological properties of these gradient copolymers were studied and are discussed in relation with their molecular structure.

pH-triggered reversible sol–gel transition in aqueous solutions of amphiphilic gradient copolymers

We demonstrate the possibility of reversible pH-controlled sol–gel transition in aqueous solution of associating amphiphilic triblock copolymer poly(styrene-grad-acrylic acid)-b-poly(acrylic acid)-b-poly(styrene-grad-acrylic acid), (PS-grad-PAA)-b-PAA-b-(PS-grad-PAA), synthesized vianitroxide-mediated (NM) radical copolymerization. The presence of pH-sensitive co-monomer units of the acrylic acid in the terminal blocks ensures the dynamic nature of the styrene-rich hydrophobic nano-domains which are formed at low pH. At small polymer concentrations the association triggered by lowering the pH gives rise to flower-like micelles stabilized by partially ionized PAA coronae. The pH-controlled association was monitored by DLS-titration and manifested in the evolution of a correlation peak in the SANS spectra. The resulting copolymer aggregates were visualized by TEM, which confirmed the spherical shape of the dense styrene-rich domains. Above the micelle overlap concentration a decrease in pH provokes macroscopic gelation. Here the styrene-reach domains perform as cross-links in the transient network. The pH-triggered sol-to-gel transition is manifested in an abrupt and strong (up to 3 orders of magnitude) increase in the zero-shear viscosity and in a characteristic change in the frequency dependence of the storage and loss moduli. The discovered effect can be used for efficient pH-control of the rheological properties of aqueous solutions.

pH Sensitive Hierarchically Self-Organized Bioinspired Films

In the present manuscript, we have demonstrated that hierarchically structured smart porous polymer films based on honeycomb-patterned surface can be elaborated from PS-b-P4VP pH-responsive block copolymer using the breath figure process. Despite the fast film formation by a bottom-up process, the copolymer nanostructuration was observed inside the walls of the honeycomb porous film. Atomic force microscopy (AFM), small angle X-ray and neutron scattering (SAXS and SANS) measurements were used to reveal both the hexagonal arrays formed by the pores at the micrometer length scale and the hexagonal copolymer self-assembly at the nanometer length scale. Contact angle (CA) measurements were used to point out the reversible pH-responsive wettability character of the surface. The PS-b-P4VP honeycomb film shows a contact angle variation of 20° between pH 9 and pH 3. An increase of the roughness was obtained with the pincushions hexagonal array enhancing the pH responsiveness of the polymer film with a switching CA gap of 75° when pH tuned from pH 9 to pH 3. This work presents the first report on honeycomb porous and pincushion films exhibiting a reversible pH-responsive character.

Synthesis and pH- and salinity-controlled self-assembly of novel amphiphilic block-gradient copolymers of styrene and acrylic acid

A novel type of amphiphilic ionic copolymer comprising a hydrophilic poly(acrylic acid) (PAA) block and an amphiphilic poly(acrylic acid)-grad-poly(styrene) (PAA-grad-PS) copolymer block was synthesized using a one step direct nitroxide-mediated polymerization (NMP). A strong influence of the macroinitiator on the values of the reactivity ratios of the co-monomers is confirmed by 1H NMR. The aggregation behaviour of the copolymers in the aqueous medium was studied by small-angle neutron scattering (SANS) and dynamic light scattering (DLS) and by transmission electron microscopy (TEM) in a wide range of pHs and ionic strengths. It has been demonstrated that PAA-b-(PAA-grad-PS) copolymers are soluble in alkaline water at room temperature without the special experimental procedures (addition of co-solvent, heating, etc.) that are usually required for solubilisation of classical PAA-b-PS diblock copolymers. The self-assembly of the PAA-b-(PAA-grad-PS) copolymers into nano-scale aggregates at low/moderate pH and/or high ionic strength was demonstrated by SANS and DLS experiments. The SANS spectra for the copolymer solution exhibit a correlation peak pointing to the formation of micelles with repulsive coronae. TEM images indicate that the micelles have an approximately spherical shape and exhibit a wide size distribution. Our results prove, that in contrast to “frozen” aggregates formed by PAA-b-PS copolymers in aqueous media, the micelles of PAA-b-(PAA-grad-PS) amphiphilic copolymers exhibit “dynamic” pH-responsive properties, i.e. they can reversibly change their aggregation number upon a variation in the pH or salinity of the solution.

Highly structured pH-responsive honeycomb films by a combination of a breath figure process and in situ thermolysis of a polystyrene-block-poly(ethoxy ethyl acrylate) precursor

In the present work, we show that Cu(0)-mediated controlled radical polymerization is a suitable method to synthesize high molar mass polystyrene-b-poly(ethoxy ethyl acrylate) PS-b-PEEA diblock copolymers. This method, applied at room temperature, is mandatory for complete preservation of ethoxy ethyl protecting groups during the course of polymerization. The synthesized PS-b-PEEA diblock copolymers were subsequently used for the elaboration of pH sensitive hierarchically structured honeycomb (HC) films through the Breath Figure (BF) process. The PS-b-PEEA hydrophobic honeycomb films were characterized by optical microscopy and atomic force microscopy (AFM) to reveal the hexagonal array of pores at the micrometer length scale, together with the phase segregation of the diblock copolymer. Similar to highly structured natural materials, the biomimetic honeycomb polymer films displayed intense iridescence. Moreover, the increase of surface roughness by peeling off the top layer of the PS-b-PEEA HC films produced superhydrophobic surfaces exhibiting a water contact angle of 155°. Subsequent deprotection of PEEA into pH-responsive poly(acrylic acid) (PAA) was performed in situ from the PS-b-PEEA honeycomb film by a simple thermolysis step carried out at 90 °C. The resulting PS-b-PAA honeycomb films showed a clear pH-responsive behavior with a water contact angle gap of 65° between a pH of 3 and 10.

Synthetic methodology effect on the microstructure and thermal properties of poly(n-butyl acrylate-co-methyl methacrylate) synthesized by nitroxide mediated polymerization

In this work, we report the synthesis of poly(n-butyl acrylate-co-methyl methacrylate) copolymers by the nitroxide mediated polymerization (NMP) technique, using N-tert-butyl-N-(1-diethylphosphono-2,2-dimethylpropyl)nitroxide (SG1) as a control agent and 2-methylaminoxypropionic-SG1 alkoxyamine (BlocBuilder®) as the initiator. The copolymers are synthesized either by batch or semi-batch processes and the gradient profile is examined via the determination of the instantaneous fraction of monomer incorporated in the copolymer. A control of the molar mass together with low molar mass distribution (Mw/Mn < 1.4) is observed. The dependence of the copolymer glass transition temperature with conversion was followed by differential scanning calorimetry. The copolymers are investigated by carbon nuclear magnetic resonance and heteronuclear multiple bond correlation (HMBC) NMR sequences to study the effect of the monomer addition mode on the microstructure of copolymers. The thermomechanical properties of gradient copolymers are finally reported to establish the effect of the composition on the mechanical behaviour of the copolymers.

Photoactive, Porous Honeycomb Films Prepared from Rose Bengal-Grafted Polystyrene

Honeycomb-structured porous polymer films based on photosensitizer-grafted polystyrene are prepared through the breath figure process. Rose Bengal (RB) photosensitizer is first attached to a well-defined poly(styrene-stat-4-vinylbenzyl chloride) statistical copolymer, synthesized by nitroxide-mediated radical polymerization. The RB grafted poly(styrene-stat-4-vinylbenzyl chloride) (ca. 20,000 g mol(-1) molar mass, 1.2 dispersity) leads to porous polymer films, with a hexagonal pore pattern, while a simple mixture of poly(styrene-stat-4-vinylbenzyl chloride) and the insoluble RB photosensitizer produced unstructured, nonporous films. The RB-grafted honeycomb films, compared with the corresponding nonporous flat films, are more efficient for oxidation of organic molecules via singlet oxygen production at a liquid/solid interface. The oxidations of 1,5-dihydroxynaphthalene to juglone and α-terpinene to ascaridole are followed in ethanol in the presence of both types of films. Oxidation of the organic molecules is a factor 5 greater with honeycomb compared to the nonporous films. This gain is ascribed to two factors: the specific location of the polar photosensitizer at the film interface and the greater exchange surface, as revealed by fluorescence and scanning electron microscopies.

Gradient or statistical copolymers by batch nitroxide mediated polymerization: effect of styrene/methyl acrylate feed

Monomer reactivity ratios in controlled radical copolymerization of styrene (S) and methyl acrylate (MA) monomers at 120 °C were determined. The Fineman–Ross method was used to calculate rs and rMA. Using this method, monomer reactivity ratio values of 0.89 and 0.22 were calculated for styrene and methyl acrylate, respectively. Because of the different reactivity ratios between the two monomers, and according to the molar fractions of two monomers, S/MA statistical or gradient copolymers were synthesized by nitroxide-mediated polymerization. Indeed, for different monomer ratios, the same statistical or gradient copolymers can be obtained by batch nitroxide mediated polymerization. These copolymers have been characterized by 1H nuclear magnetic resonance and size exclusion chromatography, and evolution of the composition has been correlated with the glass transition temperature measured by differential scanning calorimetry. Integrated intensities of the three observed peaks of (–OCH3) group in the 1H NMR spectra were used to determine the MA/MA/MA, MA/MA/S and S/MA/S triad sequences in the copolymers. Specific organization at the air/polymer interface of such copolymers has also been demonstrated by comparison between classical and attenuated total reflection (ATR) Fourier transform infra-red spectra.